Green reduction of 4-nitrophenol to 4-aminophenol in the presence of TiO2-based photocatalysts and various sacrificial agents: Dual role of 4-methoxybenzyl alcohol

Water pollution by organic chemicals poses a significant threat to the environment and human health, and 4-nitrophenol (4-NP) is one of the most persistent and toxic compounds commonly found in industrial effluents. In this study, the feasibility of 4-NP reduction to 4-aminophenol (4-AP), a high-value added chemical, has been investigated under UV light in aqueous dispersion containing different TiO2 based photocatalysts. This would, in principle, allow for simultaneous environmental remediation and production of a value-added compound. The effect of different sacrificial agents acting as hole traps, i.e. triethanolamine (TEOA), sodium sulfite (Na2SO3), sodium sulfide (Na2S) and 4-methoxybenzyl alcohol (4-MBA) on the efficiency of the reaction was systematically studied. 4-AP was successfully obtained only in the presence of TEOA and 4-MBA, highlighting the potential of these molecules as sacrificial agents. Interestingly, 4-MBA, used for the first time in this reaction, not only allowed the reduction of 4-NP to 4-AP, but its partial oxidation afforded 4-metoxybenzylaldehyde (4-MBAL), another valuable compound realizing three issues at single blow. The first issue, in fact, was the removal of the pollutant 4-NP, the second the conversion of 4-NP into a valuable compound by electron-driven reduction and the third the exploitation of holes for the partial oxidation of the sacrificial compound (4MBA) into the corresponding high-value aromatic aldehyde (4MBAL). The best results were obtained with 5 %Cu2O-P25 in the presence of 4-MBA that over 3 h of irradiation afforded 98 % 4-NP conversion with 52 % 4-AP selectivity along with 33 % 4-MBA conversion and 16 % 4-MBAL selectivity.